Filter circuits



March 13, 1934. B. LEVINE 1,951,025

FILTER CIRCUITS Filed March 14 1930 3 Sheets-Sheet 1 I 1 2-... l A 2c 05 2 C 2c I a Z Z INVENTOR Irwin B. Levine.

ATTORNEY l. B. LEVINE 1,951,025

FILTER CIRCUITS Filed March 14 1930 3 Sheets-Sheet 2 March 13, 1934.

[r win 5 Levine.

A'IITORNEY.

Patented Mar. 13, 1934 UNITED STA-res PATENT OFFICE FILTER CIRCUITSIrwin B. Levine, Newark, N. J assignor to Wired Radio, Inc., New York,N. Y., a corporation of Delaware Ashrams Maia 14", 1930, Serial Nb.435,694.

1 Claim. 01. 178-44) ing.1the1.transfer of energy-through band passfilters having terminal impedances of unequal magnitudes. 1 A l Another.object of my invention is to produce a filter adapted to operate-withmaximum efficiency between 1 unequalimpedances.

Another object contemplated by my invention consists in producinganovel-filter adapted to 15 transfer 1.energ y. with maximum efficiencybetweenunequal impedances-and yet constructed of, few and simple partsto -thuseffect an appreciablereduction in the present cost of manu- 1facture ofsuch filters. 1 1 f 1 Other desirable features will behereinafter pointed out and discussed Some type of wave filter isusually employed whenever -it is desiredto-transmit an electric currentof a givenband of frequencies, to attenuate all frequencies above andbelow the range which it is desired to transmit. Many types of filtershave'been. used for this purpose. One type j consisting of positive andnegative reactances, successive meshes of which are coupled together 3by electromagnetic inductance only,-is described in HeavisidesElectrical Circuit Theory by a Dr.

Louis Cohen. The use of such a wavefilter in connecting impedances oflike value is well known M in the art.

that the output impedance of the sendingorganization connected in thefilter, the characteristic impedance of the filter, and the input 1m 40pedance of the receiving circuit to which the usually happens inpractical applications that the sending and receiving circuits havecharacteristic impedances which are not equal but differ may be desiredto transfer power through a filter from the anode circuit of athermionic tube to According to magnetic theory,

and with infinite mutual reactance by means of To prevent reflectionlosses it is imperative filter is connected, shall be equal. It,however,

from each other by wide margins. For example, it,

e e i ideal transformer having no leakage reactance,

which a transformation of impedance is possible, such transformationbeing equal to the ratio of the primary impedance to the secondary impedance. Obviously, such a transformer could be used in the aboveexample to efliciently connect the unequal impedances. In actualpractice, thedesign of such a transformer. is quite impossible and, infact, cannot even be approximatedat relative high frequencies. 1

To accomplish the transfer of maximum energy between unequal impedances,it has heretofore been the. practice to utilize a filter networkwhichincreased greatly in complexity in the effort to obtain maximum transfereificiency at a greater number of frequencies, thus increasing the costof manufacture of such filters to an undesirable point. 1

It is the purpose of my invention to provide simple and effective meansfor accomplishing the 11; maximum transfer of energy between unequal im-1 pedances at all frequencies which may be transmitted by a wave filter.

In the drawingsaaccompanying and forming a part of this specification: 11 Fig. 1 shows diagrammatically a wave. filter of the electromagneticcoupled type, the impedances of which are designed to transmit freelyone band of frequencies between impedances of equal values. Fig. 2illustrates diagrammatically two filters, the characteristic impedancesonly of which differ from each-other, connected by a transformer ofideal characteristics. 1. L

Figs. 3-5 diagrammatically illustrate the steps in the process oftranslating the filters shown in Fig. 2 and the hypothetical transformerbetween them into my novel equivalent practical network. Figs. 6 and 7show diagrammatically two practical applications of thetransformation ofimpedance in accordance with my invention. W 5

Referring now in particular to Figsl and}, Fig, l illustrates'a type ofelectromagnetically coupled filter described in the publication ofDr.Louis lohen hitherto mentioned herein, adapted to unite impedances ofequal value and of a value equal to that of thecharacteristic impedanceof the filter. It is thegeneral type of elecw tromagnetically coupledfilter which I propose .to improve and modify in carrying out myinvention. 1 d 1 Referring ,to. Fig. ,2, filter A, has .identically.the1same characteristics asfilter B, exceptthat the characteristicimpedances of the filters differ 1 from each other by a ratio which willbe hereinfilter A is;

after discussed. In the figure K designates the coupling coefiicientbetween meshes,

the self-inductance of the coupling coil and C the capacitance of awhole mesh, the one half mesh capacitance being 20.

In filter A, the lower cut-off frequency is;

which may be a frequency between the upper and lower cut-off frequenciesis;

i2 1 A cosh- (w Where and w=21rf, where f is any frequency for which theattenuation A is being computed. The characteristic impedance at anyfrequency Z for 1 (.0 60 K 2- Z5 2 1 0 0 l In B, the lower cut-offfrequency is;

The upper cut-off frequency is;

1 27r' /fi1/1K The attenuation constant for any frequency which may be afrequency between the upper and illower cut-01f frequencies is;

The characteristic impedance at any frequency 2 for filters B is;

Z =i/ 2 8B ZwC 1 e0 wo K] 2 Z5 =Zg 8 The characteristic impedance offilter B is equal to. the characteristic impedance of filter Amultiplied by the factor 0. Since at every frejquency the ratio ofimpedances of the filters is equal to a an ideal transformer of ratiolra may be imagined between the'two filters and then the impedancespreceding and following the transformer changed in such manner as toreali ize practically all of the functions of the imaginary idealtransformer.

Referring now specifically to Figs. 3-5, I propose to illustrategraphically the transformation of the ideal organization showndiagrammatically in Fig. 2 into a practical equivalent. The first stepis shown in Fig. 3 which shows a II network which is completelyequivalent to the condensers 2G in series with an ideal transformer ofratio Iza.

Fig. 4 diagrammatically indicates a T network which is equivalent to thetwo coupling coils, each having a self-inductance of and a couplingcoefficient between the coils of K. These equivalences are well known inthe art.

Fig. 5 shows diagrammatically a T network which is the exact equivalentof the T network of Fig. 4 in combination with the 11 network of Fig. 3

' and the condenser 2 of filter B, Fig. 2.. Fig. 5 further illustratesby successive transformations of 11 networks into T networks, whichtransformations are all well known in the art, and each networksuccessively operated on being indicated by a dotted circle, how thecombination is finally resolved into a Wave filter in all respectssimilar in type to that shown in Fig. 1, but in which at the point wherea change of impedance by a factor as a is desired allsucceedingimpedances are multiplied by the factor o The change inimpedance is always effected at a point where two or more coils arecoupled by electromagnetic induction. One or more coils may beassociated with a wave filter mesh of characteristic impedance given byone equation, and one or morecoils may be associated with a wave filtermesh of a characteristic impedance given by the same equation andmultiplied by the factor a By following the same procedure, it will beobvious that a transformation of characteristic impedance may beeffected as many times as there are meshes coupled by electromagneticinduction to effect any desired transformation of impedance and topermit the output circuits of organizations being connected to the inputcircuit of other organizations having impedances of widely differentvalues to permit of the transfer of energy from one organization toanother through the filter with minimum reflection losses and hence withmaximum efficiency.

In order to completely disclose my invention, I have shown in Figs. 6and 7 two practical examples giving, therefore, the actual magnitudes ofthe component parts.

It was desired in the first example shown in Fig. 6 to provide a bandpass filter to have a characteristic impedance of 50 ohms at its meanfrequency and to transmit freely a band of frequencies from 44kilocycles to 53 kilocycles. The filter is intended to work into a linewhose impedance is 10 ohms at the mean frequency of 48 kilocycles and totransmit the maximum energy at the mean frequency. The completepractical design of the two preceding meshes and the final I half meshwith a transformed impedance is shown in Fig. 6.

The example shown in Fig. 7 illustrates the solution of the problem ofproviding a filter having an impedance of 50 ohms at its mean frequency1 and adapted to connect a transmitting amplifier into a line totransmit with maximum efficiency a band of frequency from 44 kilocyclesto 53 kilocycles, the line impedance being of the value of 50 ohms. Inthis design, the condenser C has a value of .00615 microfarads in eachmesh but one where it was found inconvenient to place such a condenserand a .005 microfarad condenserwas substituted. The two inductancesassociated with the substituted condenser were lliil changed from .902millihenrys to 1.108 millihenrys each as shown in the figure. In themesh containing the .005 condenser, the characteristic impedance was ata point A-A stepped up by a factor of 1.229 and stepped down by the samefactor at point BB. No reflection loss or other change of filtercharacteristics were noted, thus illustrating one of the desirablefeatures of my invention, that is, the increased latitude of selectionin component parts, thus simplifying and cheapening manufacture.

It will be noted from the above that my improved filter provides asimple convenient means for transforming impedance to permit of theconnection by a filter of unequal terminal impedances with minimumreflection losses, that such filter lends itself readily to cheapmanufacture and that it admirably accomplishes the purpose of affordingmeans for transferring energy between terminal impedances of unequalmagnitude at maximum efliciency.

Having thus completely described my invention what I claim as new anddesire to secure by United States Letters Patent is as follows:

A transmitting network adapted to connect a transmitting organizationwith a receiving organization in which transfer of energy isaccomplished by meshes composed of positive and negative reactancesconnected by electromagnetic induction only, and in which the first halfmesh and the last half mesh only are matched as to their characteristicimpedances with the transmitting and receiving organizations,respectively, and in which the intermediate meshes have the samearbitrarily chosen impedance characteristics.

IRWIN B. LEVINE.

